86 
Proceedings of Royal Society of Edinburgh. [sess. 
principles. The isatmid lines show the transparency of the air for 
winds from different directions and for different degrees of humidity. 
It must also be clearly understood that this map only refers to 
Talkirk, or some place near it. Every place has its own isatmid 
chart, the lines of which are determined by the density of the 
population surrounding it and the direction in which the populated 
areas are situated. 
The curves in the Plate were drawn in the following manner : — 
Lines representing the directions of the wind were drawn through 
the round black spot representing the position of Falkirk on the 
map. On the lines so drawn were marked off in miles, using the 
same scale as the map, the mean limit of visibility of the winds 
from the different directions when the wet bulb depression was 2°, 
the figures being taken from Table JSTo. IV. Through the points so 
obtained the isatmid line for 2° was drawn. The same thing was 
then done for the observations at the other wet bulb depressions. 
This has not been done for each degree of difference of wet bulb de- 
pression, but only for every 2 degrees. If we had used the figures in 
Table No. I. for drawing the isatmid lines, we should have obtained 
a rather unsatisfactory and somewhat confusing set of lines. As 
will be seen from the figures, some of the curves would have crossed 
each other, owing to the variation in the limit of visibility not 
rising regularly with the dryness, though it is in the highest degree 
probable that it ought to do so. This crossing is probably due 
to there being too few observations to give a good mean^ and also to 
1° of difference in wet bulb depression being rather small to give a 
decided difference in the transparency. A much better result is 
obtained by combining the figures for two degrees of wet bulb 
depression. This has been done, and Table IV. is the result of 
condensing the observations in Table No. I. In Table IV. the first 
column gives, as in the other tables, the direction of wind, the next 
two columns the number of observations and the limit of visibility 
when the wet bulb depression was 2° ; in the next two columns 
are entered the number of observations and the limit of visibility 
when the depression was 3° and 4° ; in the next two columns are 
combined the observations when the depression was 5° and 6° ; and 
in the last two the observations when it was 7° and 8°. If we now 
examine the figures in this table we shall find that, with the excep- 
